Poly(ADP-ribose) polymerases (PARPs), nuclear enzymes found in almost all eukaryotic cells, catalyze the transfer of ADP-ribose units from nicotinamide adenine dinucleotide (NAD+) to nuclear acceptor proteins, and are responsible for the formation of protein-bound linear and branched homo-ADP-ribose polymers. Activation of PARP and resultant formation of poly(ADP-ribose) can be induced by DNA strand breaks after exposure to chemotherapy, ionizing radiation, oxygen free radicals, or nitric oxide (NO).
Because this cellular ADP-ribose transfer process is associated with the repair of DNA strand breakage in response to DNA damage caused by radiotherapy or chemotherapy, it can contribute to the resistance that often develops to various types of cancer therapies. Consequently, inhibition of PARP may retard intracellular DNA repair and enhance the antitumor effects of cancer therapy. Indeed, in vitro and in vivo data show that many PARP inhibitors potentiate the effects of ionizing radiation or cytotoxic drugs such as DNA methylating agents. Therefore, inhibitors of the PARP enzyme are useful as cancer chemotherapeutics.
In addition, it has been shown that inhibition of PARP promotes resistance to brain injury after stroke (Endres et al., “Ischemic Brain Injury is Mediated by the Activation of Poly(ADP-Ribose)Polymerase,” J. Cerebral Blood Flow Metab. 17:1143-1151 (1997); Zhang, “PARP Inhibition Results in Substantial Neuroprotection in Cerebral Ischemia,” Cambridge Healthtech Institute's Conference on Acute Neuronal Injury: New Therapeutic Opportunities, Sep. 18-24, 1998, Las Vegas, Nev.). The activation of PARP by DNA damage is believed to play a role in the cell death consequent to stroke, head trauma, and neurodegenerative diseases. DNA is damaged by excessive amounts of NO produced when the NO synthase enzyme is activated as a result of a series of events initiated by the release of the neurotransmitter glutamate from depolarized nerve terminals (Cosi et al., “Poly(ADP-Ribose) Polymerase Revisited: A New Role for an Old Enzyme: PARP Involvement in Neurodegeneration and PARP Inhibitors as Possible Neuroprotective Agents,” Ann. N.Y. Acad. Sci., 366-379). Cell death is believed to occur as a result of energy depletion as NAD+ is consumed by the enzyme-catalyzed PARP reaction. Therefore, inhibitors of the PARP enzyme are useful inhibitors of neurotoxicity consequent to stroke, head trauma, and neurodegenerative diseases.
Further, inhibition of PARP should be a useful approach for treatment of conditions or diseases associated with cellular senescence, such as skin aging, through the role of PARP in the signaling of DNA damage. See, e.g., U.S. Pat. No. 5,589,483, which describes a method to extend the lifespan and proliferative capacity of cells comprising administering a therapeutically effective amount of a PARP inhibitor to the cells under conditions such that PARP activity is inhibited. Hence, inhibitors of the PARP enzyme are useful therapeutics for skin aging.
In yet a further application, PARP inhibition is being explored at the clinical level to prevent development of insulin-dependent diabetes mellitus in susceptible individuals (Saldeen et al., “Nicotinamide-induced apoptosis in insulin producing cells in associated with cleavage of poly(ADP-ribose) polymerase,” Mol. Cellular Endocrinol. (1998), 139:99-107). PARP inhibitors should therefore be useful as diabetes-prevention therapeutics.
PARP inhibition is also an approach for treating inflammatory conditions such as arthritis (Szabo et al., “Protective effect of an inhibitor of poly(ADP-ribose) synthetase in collagen-induced arthritis,” Portland Press Proc. (1998), 15:280-281; Szabo, “Role of Poly(ADP-ribose) Synthetase in Inflammation,” Eur. J. Biochem. (1998), 350(1): 1-19; Szabo et al., “Protection Against Peroxynitrite-induced Fibroblast Injury and Arthritis Development by Inhibition of Poly(ADP-ribose) Synthetase,” Proc. Natl. Acad. Sci. USA (1998), 95(7):3867-72). PARP inhibitors are therefore useful as therapeutics for inflammatory conditions.
Inhibition of PARP has usefulness for protection against myocardial ischemia and reperfusion injury (Zingarelli et al., “Protection against myocardial ischemia and reperfusion injury by 3-aminobenzamide, an inhibitor of poly (ADP-ribose) synthetase,” Cardiovascular Research (1997), 36:205-215). Therefore, PARP inhibitors are useful in therapy of cardiovascular diseases.
The PARP family of enzymes is extensive. It has recently been shown that tankyrases, which bind to the telomeric protein TRF-1, a negative regulator of telomere length maintenance, have a catalytic domain that is strikingly homologous to PARP and have been shown to have PARP activity in vitro. It has been proposed that telomere function in human cells is regulated by poly(ADP-ribosyl)ation. PARP inhibitors have utility as tools to study this function. Further, as a consequence of regulation of telomerase activity by tankyrase, PARP inhibitors should have utility as agents for regulation of cell life-span, e.g., for use in cancer therapy to shorten the life-span of immortal tumor cells, or as anti-aging therapeutics, since telomere length is believed to be associated with cell senescence.
Competitive inhibitors of PARP are known. For example, Banasik et al. (“Specific Inhibitors of Poly(ADP-Ribose) Synthetase and Mono(ADP-Ribosyl)transferase,” J. Biol. Chem. (1992) 267: 1569-1575) examined the PARP-inhibiting activity of 132 compounds, the most potent of which were 4-amino-1,8-naphthalimide, 6(5H)-phenanthridone, 2-nitro-6(5H)-phenanthridone, and 1,5-dihydroxyisoquinoline. Griffin et al. reported the PARP-inhibiting activity for a series of benzamide compounds (U.S. Pat. No. 5,756,510; see also “Novel Potent Inhibitors of the DNA Repair Enzyme poly (ADP-ribose)polymerase (PARP),” Anti-Cancer Drug Design (1995), 10:507-514) and quinalozinone compounds (International Publication No. WO 98/33802). Suto et al. reported PARP inhibition by a series of dihydroisoquinoline compounds (“Dihydroisoquinolines: The Design and Synthesis of a New Series of Potent Inhibitors of Poly(ADP-ribose) Polymerase,” Anti-Cancer Drug Design (1991), 7:107-117). Griffin et al. have reported other PARP inhibitors of the quinazoline class (“Resistance-Modifying Agents. 5. Synthesis and Biological Properties of Quinazoline Inhibitors of the DNA Repair Enzyme Poly(ADP-ribose) Polymerase (PARP),” J. Med. Chem., ASAP Article 10.1021/jm980273t S0022-2623(98)00273-8; Web Release Date: Dec. 1, 1998).
Nonetheless, there is still a need for small-molecule compounds that are potent PARP inhibitors, especially those that have physical and chemical properties desirable for pharmaceutical applications.